French patent No. 1 391 661 describes a bundle of tubes of this type.
In such a bundle, there exists a tube-free notch of triangular shape between each radiating spike constituting a tube-containing zone. Such a notch is necessary to allow steam to pass towards the tubes of two spikes situated on either side of the notch. A notch can thus be said to "feed" the tubes in two half-spikes on either side of the notch.
The total cross-section occupied by a bundle is a function of:
the section of the tube-containing zone which depends on the number of tubes and on the pitch of the tubes; PA1 the section required for said notches that pass steam between the spikes; and PA1 the section of the tube-free zones that result from constraints on the trace: e.g. around the tube-containing zone referred to as the "air cooler".
For a given constant speed V in meters per second (m/s) of the steam along a flow notch between two spikes, a given length L in meters (m) of the tubes, and a given flow rate q.sub.i in cubic meters per second (m.sup.3 /s) of steam condensed per tube, which is the same for all of the tubes, it can be shown by calculation that the section S required for a triangular notch is equal to 1/2H (where l is the width of the notch: i.e. the distance between two spikes at their ends, and where H is the height of the notch, corresponding to the height of the two half-spikes on either side of the notch), and is also given by S=NH(q.sub.i /2LV) where N=the number of tubes in the two half-spikes fed by the triangular notch of height H.
It can thus be seen that the section required for a notch, which is a tube-free area required for passing steam towards the two half-spikes on either side of the notch, is proportional to the number N of tubes in the two adjacent half-spikes and to the height H of the notch.
It is therefore clear that large bundles having long spikes, and thus long notches, require a greater tube-free area per tube than do smaller bundles having shorter spikes.
Thus, for given steam speed V, the number of tubes that can be installed per unit area decreases with increasing height H of the triangular steam notches.
Unfortunately, the power exchanged in a bundle of tubes is proportional to the heat exchange surface area, and thus to the number of tubes.